Allistair, The Mendix Platform doesn't support the usage of 'attributeGroup' at this time. No matter where you use the attribute group it will not show the reference attributes.
Please enter a ticket at support.mendix.com containing the same information you have provided here. (The xsd, xml, and what exactly you are missing). Our Support and R&D team will evaluate your request and schedule this for a future release.
In the mean time you could move forward and make a few tweaks to the XSD. Since the attribute group is basically an XML snippet that can be reused throught the XSD. You could replace all attribute groups by the attributes it is referencing. This will at least allow you to move forward.
Your UniqueID_Type would look like this:
<xs:complexType name="UniqueID_Type">
<xs:sequence>
<xs:element name="CompanyName" type="xs:string" minOccurs="0">
</xs:element>
</xs:sequence>
<xs:attribute name="URL" type="xs:string">
</xs:attribute>
<xs:attribute name="Type" type="OTA_CodeType" use="required">
</xs:attribute>
<xs:attribute name="Instance" type="xs:string">
</xs:attribute>
<xs:attribute name="ID" type="xs:string" use="required">
</xs:attribute>
<xs:attribute name="ID_Context" type="xs:string" use="optional">
</xs:attribute>
</xs:complexType>